1. Field of the Invention
This invention relates in general to the art of noise suppression by a summation process and, more particularly, to a method and apparatus especially adapted to remove interfering, continuously-changing, multi-frequency periodic noise signals from contaminated seismic signals while they are being detected during seismic exploration.
2. Description of the Prior Art
A seismic signal detected from the earth includes: (1) a desired seismic component having amplitude, phase and frequency variations representing seismic information, (2) an undesirable multi-frequency, electrical power-grid-related noise component, and (3) other such environmental and instrument-related noise components.
As is well known, in a seismic data acquisition channel, the amplitude of the noise component can be orders of magnitude larger than the amplitude of the seismic component. This fact imposes a reduction in the channel's amplifier gains, resulting in a reduced dynamic range and, hence, a decreased resolution for the overall seismic signal. Such self-imposed reduced gain causes the unavoidable self-generated noise within the channel's amplifiers to increase the channel's noise-to-signal ratio.
Therefore, unless noise is effectively reduced from the contaminated seismic signal, the quality of the seismic data being acquired during seismic exploration and its subsequent interpretation will deteriorate.
The oldest and yet still the most widely used noise suppressor for seismic channels is the notch filter. Periodic noise becomes attenuated according to the depth and width of the notch filter whose center frequency is nominally set to the fundamental frequency of the predominant noise signal, typically the 60 Hz frequency of the local power grid.
In many geographic areas, however, the second or third harmonic of the 60 Hz signal is the predominant noise frequecy. As a consequence, a distinct notch filter is required for each harmonic or subharmonic of the predominant fundamental frequency. Hereinafter, the fundamental frequency and its harmonics and/or subharmonics will be collectively called in short the "noise signal".
Narrow-band analog filters are expensive and difficult to build because they require closely matched components. For this practical reason, most notch filters have a relatively wide bandwidth, say 10 Hz, and cause undesirable phase shifting characteristics in the seismic signal being processed therethrough.
U.S. Pat. No. 3,704,444 describes a seismic channel which utilizes its digital data processor network to perform a double function: (1) to process the digitized seismic data in the seismic channel, and (2) to produce a feedback error signal which is subtracted from the incoming analog seismic signal at the input to the seismic channel. It is believed that such a system, because of its inherent structural limitations, would lack flexibility in adapting to multiple and changing noise frequencies. Therefore, such a system even at best would appear to be the equivalent of a notch filter which is suitable for only a single dominant frequency.
The above described and other well-known problems, associated with notch filters or their equivalents, have prompted attempts to optimize the entire data acquisition instrumentation within the seismic channel in the hope of being able to eliminate notch filters altogether.
One such approach uses a "line balancer" which usually takes the form of a bridge having a pair of variable impedance arms that are electrically connected between each wire within the seismic cable and ground. These impedances are adjusted to increase the common mode noise rejection at the dominant noise frequency, in order to obtain less contamination of the difference mode signal by the common mode noise signal.
But, working with a large number of line balancers requires tedious and very time-consuming manual adjustments. Also, because ambient noise tends to change continuously, the first-adjusted balancer will require readjustment after the last balancer is adjusted and before seismic energy becomes injected into the ground. Such a procedure can become a never-ending task, especially for a seismic data gathering system having 100 or more seismic channels.
The common mode attenuator is a more recent derivative of the line balancer. It requires amplifying the common mode signal with two separate amplifiers. One amplified signal is applied to one wire of the seismic cable through a resistor, and the other signal is applied to the same wire through a capacitor. The outputs of the two amplifiers are also inverted. One inverted signal is applied to the other wire of the seismic cable through a resistor, and the other inverted signal is also applied to the other wire through a capacitor. The gains for these two common mode amplifiers are determined by correlating the difference mode signal with the common mode signal in order to balance the impedance of the seismic cable at the dominant correlated frequency. But, because the impedance of a seismic cable constitutes a complex quantity, varying with frequency, temperature, etc., the cable will still remain unbalanced for frequencies other than the correlated frequency.
It has also been proposed to synthesize a nulling signal having a predetermined, fixed, single frequency which is substantially equal to the fundamental frequency of the anticipated periodic noise signal. This nulling signal is manually adjusted to have the desired amplitude and phase. Even at best, only the anticipated predominant fundamental noise frequency may be suppressed with such a nulling signal.
In sum, the known noise-suppression systems still have serious drawbacks, such as: (1) introduction into the seismic channel of substantial distortions in the phase, frequency, and/or amplitude characteristics of the seismic signal being gathered; (2) need for time-consuming manual adjustments; and (3) noise cancellation limited to only a single anticipated predominant fundamental noise frequency.
Effective and practical noise suppression must be capable of removing the harmful effects produced by an unanticipated dominant noise frequency and/or its harmonics and subharmonics with a minimum amount of seismic signal degradation. For example, a notch filter, even when well tuned, attenuates the 60 Hz power grid noise by about 40 dB, but does not suppress any harmonics and/or subharmonics thereof. At the same time, the notch filter introduces an undesired phase reversal at the center frequency and modifies the amplitudes of the filtered seismic signals over a bandwidth of about 10 Hz on either side of its center frequency.
It is a broad object of the present invention to provide a novel and effective noise suppressor, which is able to sufficiently suppress the harmful effects on the desired seismic signal caused by a continuously-changing, multi-frequency noise signal that contaminates the seismic signal component as it is being gathered within each one of the seismic channels during seismic exploration of the earth.
It is a further object to provide such noise suppression without at the same time introducing into the seismic channel substantial distortions in the phase, frequency, and/or amplitude characteristics of the seismic signal being gathered and without requiring time-consuming manual adjustments to the networks within the noise suppression system.